The man who could be Germany’s vice-chancellor within weeks on Thursday called for the European Union to transform itself into a “United States of Europe”.

Martin Schulz, the leader of the German Social Democratic Party (SPD), called for a new federal constitution for the EU by 2025.

Hours before his party voted to open talks on forming a new coalition with the beleaguered Angela Merkel, Mr Schulz made clear he would demand radical EU reform and far deeper integration than previously envisaged as his price for ending weeks of political crisis in Germany.

“I want a new constitutional treaty to establish the United States of Europe. A Europe that is no threat to its member states, but a beneficial addition,” he said in a speech to his party conference.

Under Mr Schulz’s proposals, Brussels would be given power over individual member states’ foreign and domestic policy, as well as taxes.

Countries who refuse to sign up to a new federal Europe should automatically lose their EU membership, he said.

A deal with Mr Schulz’s party is Mrs Merkel’s last chance to prevent new elections after coalition negotiations with smaller parties broke down last month.

Urging delegates to vote in favour of talks with Mrs Merkel’s Christian Democrats (CDU), Mr Schulz said he would make a “complete turnaround in Germany’s European policy” a central condition of any deal.

But Mrs Merkel lost no time in rejecting Mr Schulz’s proposals for a federal Europe, ruling them out before delegates had even finished voting.

“I believe the ability to act now is the priority, not setting long-term goals,” she said.

More cooperation between EU members was more important than drawing up new constitutions and taking powers from member states, she argued.

“We have to be economically strong, we need to work better together in the field of defence. We must pursue a common foreign policy, a common development policy, to be taken seriously as a continent or as a European Union.

“That’s why my goal for 2025 is better cooperation on defence and these other issues.”

The threat of new German elections receded after the SPD on Thursday voted in favour of “open-ended talks” with Mrs Merkel’s party.

Negotiations are not expected to begin until January, and the options of a new coalition and the SPD supporting a Merkel-led minority government both remain on the table.

Mr Schulz’s speech marked a dramatic attempt to seize back the initiative after he was forced to back down over his earlier refusal to open talks with Mrs Merkel by a rebellion in his parliamentary party.

A former president of the European parliament and committed federalist, he appears to have seized on Europe as a new central cause for the SPD after it suffered its worst ever election result in September.

His proposals are a bid to put himself and Germany alongside France’s Emmanuel Macron as standard-bearers for a Europe of ever closer union.

It is a move that will pit France and Germany against central European countries. While the EU maintains a united front over Brexit, there are deep divisions between east and west over the handling of the migration crisis and the state of the rule of law and democracy in Hungary and Poland.

Mr Schulz acknowledged as much in his speech, accusing Poland of “systematically undermining European values”, and saying Hungary was “increasingly isolating itself”.

Once drafted, a new federal constitution would be “presented to the member states, and those countries who are against it will simply leave the EU,” he said — remarks clearly aimed at the reluctant central Europeans.

The EU’s uneasy relationship with Hungary, Poland and the Czech Republic sunk to a new low on Thursday as Brussels said it would take them to the European Court of Justice (ECJ) for refusing to accept the bloc’s imposed quotas of migrants.

EU countries agreed to redistribute 120,000 asylum-seekers from Italy and Greece in 2015. The three countries, outvoted over the compulsory law, now face huge, daily fines unless they take their share.

“There can be no free riders in serious crises,” said Frans Timmermans, European Commission Vice-President. “We must be able to depend on every single member state.”

“It is an infringement of our sovereignty that we should not be allowed to decide whom we are going to live with,” Peter Szijjarto, the Hungarian foreign minister said.

“Nothing has changed, our position remains the same. We do not agree with the relocation decisions”, Witold Waszczykowski, the Polish foreign minister said.

Hungary was also hit with a separate EU lawsuit on Thursday over a higher education law that could close a university founded by George Soros, while Poland has been at loggerheads with Brussels over EU concerns over reforms to its judicial system.

The future of Poland's Prime Minister, Beata Szydlo, hung in the balance Thursday as top political leaders considered a government reshuffle that could see her replaced.

Jaroslaw Kaczynski, who in outlandish fashion has accused European Council President Donald Tusk of plotting the murder of his twin brother, leads the ruling Law and Justice Party and is thought to advocate the change.

Were Kaczynski, a former prime minister, to put himself or a similarly Eurosceptic candidate forward, relations with Brussels would likely worsen.